1. Field of the Invention
The present invention relates to a TDMA (Time Division Multiple Access) satellite communication system for transmitting serial subsignal data, and more particularly to a data transmitting system for transmitting low speed data between stations, between which a high-speed main signal line has been established for TDMA satellite communication.
2. Description of the Related Art
In a satellite communication network, there may be a need to transmit a small amount of subsignal data other than main signal data. The small amount of data may be necessary depending on the operation of a station such as a reference station or a slave station from which the small amount of data is transmitted or by which the small amount of data is received. The small amount of data may be a message generated discontinuously with a non-fixed bit rate. Such small amount of data must be transmitted with a small error rate. Therefore, the small amount of data must be transmitted through a small-capacity data line separate from the main line. Note that the small-capacity data line and the main line in this description are not the real lines but are time slots allocated to a multiplexed data line.
To provide such a small-capacity data line between stations in a frame format including burst data and burst signals for satellite communication, conventionally, a special position for the small-capacity data line is previously allocated, as a burst, for the small-capacity data line, and the previously provided burst small-capacity data line is accessed when necessary by a reference station or a slave station during communication between the reference station and the slave station or between slave stations.
In the conventional system, since the special position for the small-capacity data line is fixed and constantly allocated in the frame format, a percentage of the transfer rate is occupied by the small-capacity data line regardless of whether or not the the special small-capacity data line is used, so that the frame efficiency, which is the ratio between the main signal data rate and the transfer rate, is lowered. In addition, the reliability of the small-capacity data line thus allocated is not high because the error rate of the small-capacity data is almost the same as the error rate of the main signal data. Therefore, when a high-reliability line is required for checking a connection of the main-signal line for example, additional protecting means such as a means for repeating the transmission, a means for receiving a decision concerning reliability of transmission, and so forth must be provided. This is a problem in the conventional art.